Angiogenesis, the process of new blood vessel development and formation, plays an important role in numerous physiological events, both normal and pathological. Angiogenesis occurs in response to specific signals and involves a complex process characterized by infiltration of the basal lamina by vascular endothelial cells in response to angiogenic growth signal(s), migration of the endothelial cells toward the source of the signal(s), and subsequent proliferation and formation of the capillary tube. Blood flow through the newly formed capillary is initiated after the endothelial cells come into contact and connect with a preexisting capillary.
The naturally occurring balance between endogenous stimulators and inhibitors of angiogenesis is one in which inhibitory influences predominate. Rastinejad et al., 1989, Cell 56:345-355. In those rare instances in which neovascularization occurs under normal physiological conditions, such as wound healing, organ regeneration, embryonic development, and female reproductive processes, angiogenesis is stringently regulated and spatially and temporally delimited. Under conditions of pathological angiogenesis such as that characterizing solid tumor growth, these regulatory controls fail.
Unregulated angiogenesis becomes pathologic and sustains progression of many neoplastic and non-neoplastic diseases. A number of serious diseases are dominated by abnormal neovascularization including solid tumor growth and metastases, arthritis, some types of eye disorders, and psoriasis. See, e.g., reviews by Moses et al., 1991, Biotech. 9: 630-634; Folkman et al., 1995, N. Engl. J. Med., 333:1757-1763; Auerbach et al., 1985, J. Microvasc. Res. 29:401-411; Folkman, 1985, Advances in Cancer Research, eds. Klein and Weinhouse, Academic Press, New York, pp. 175-203; Patz, 1982, Am. J. Opthalmol. 94:715-743; and Folkman et al., 1983, Science 221:719-725. In a number of pathological conditions, the process of angiogenesis contributes to the disease state. For example, significant data have accumulated which suggest that the growth of solid tumors is dependent on angiogenesis. Folkman and Klagsbrun, 1987, Science 235:442-447.
The maintenance of the avascularity of the cornea, lens, and trabecular meshwork is crucial for vision as well as to ocular physiology. There are several eye diseases, many of which lead to blindness, in which ocular neovascularization occurs in response to the diseased state. These ocular disorders include diabetic retinopathy, neovascular glaucoma, inflammatory diseases and ocular tumors (e.g., retinoblastoma). There are also a number of other eye diseases which are also associated with neovascularization, including retrolental fibroplasia, uveitis, retinopathy of prematurity, macular degeneration, and approximately twenty eye diseases which are associated with choroidal neovascularization and approximately forty eye diseases associated with iris neovascularization. See, e.g., reviews by Waltman et al., 1978, Am. J. Ophthal. 85:704-710 and Gartner et al., 1978, Surv. Ophthal. 22:291-312. Currently, the treatment of these diseases, especially once neovascularization has occurred, is inadequate and blindness often results. Studies have suggested that vaso-inhibitory factors which are present in normal ocular tissue (cornea and vitreous) are lost in the diseased state.
An inhibitor of angiogenesis could have an important therapeutic role in limiting the contributions of this process to pathological progression of the underlying disease states as well as providing a valuable means of studying their etiology. For example, agents that inhibit tumor neovascularization could play an important role in inhibiting metastatic tumor growth.
The components of angiogenesis relating to vascular endothelial cell proliferation, migration and invasion, have been found to be regulated in part by polypeptide growth factors. Experiments in culture, indicate that endothelial cells exposed to a medium containing suitable growth factors can be induced to evoke some or all of the angiogenic responses. Several polypeptides with in vitro endothelial growth promoting activity have been identified. Examples include acidic and basic fibroblast growth factors, transforming growth factors .alpha. and .beta., platelet-derived endothelial cell growth factor, granulocyte colony-stimulating factor, interleukin-8, hepatocyte growth factor, proliferin, vascular endothelial growth factor and placental growth factor. See, e.g., review by Folkman et al., 1995, N. Engl. J. Med., 333:1757-1763.
Although extracts from several different tissue sources have been shown to contain anti-angiogenic activity, several molecules such as platelet factor-4, thrombospondin, protamine, and transforming growth factor B, have been found to negatively regulate different aspects of angiogenesis, such as cell proliferation or cell migration, no single tissue-derived macromolecule capable of inhibiting angiogenesis has been identified in the prior art. See, e.g., reviews by Folkman, J., 1995, N. Engl. J. Med. 333:1757-1763 and D'Amore, 1985, Prog. Clin. Biol. Res. 221:269-283. There is therefore a great need for the further identification and characterization of chemical agents which can prevent the continued deregulated spread of vascularization and which would potentially have broad applicability as a therapy for those diseases in which neovascularization plays a prominent role.
Capillary endothelial cells ("EC") proliferate in response to an angiogenic stimulus during neovascularization. Ausprunk and Folkman, 1977, J. Microvasc. Res. 14:153-65. An in vitro assay assessing endothelial cell proliferation in response to known angiogenesis simulating factors, such as acidic or basic fibroblast growth factor (aFGF and bFGF, respectively), has been developed to mimic the process of neovascularization in vitro. This type of assay is the assay of choice to demonstrate the stimulation of capillary EC proliferation by various angiogenic factors. Shing et al., 1984, Science 223:1296-1298.
The process of capillary EC migration through the extracellular matrix towards an angiogenic stimulus is also a critical event required for angiogenesis. See, e.g., review by Ausprunk et al., 1977, J. Microvasc. Res. 14:53-65. This process provides an additional assay by which to mimic the process of neovascularization in vitro. A modification of the Boyden chamber technique has been developed to monitor EC migration. Boyden et al., 1962, J. Exptl. Med. 115:453-456, Example 4. To date, only a few tissue-derived EC cell migration inhibitors are known. See, e.g., review by Langer et al., 1976, Science 193:70-72.
In the early 1970's, a number of in vivo angiogenesis model bioassays were widely used. These model systems included rabbit corneal pocket, chick chorioallantoic membrane ("CAM"), rat dorsal air sac and rabbit air chamber bioassays. For review, see, Blood et al., 1990, Biochem. et Biophys. Acta 1032:89-118. The development of controlled release polymers capable of releasing large molecules such as angiogenesis stimulators and inhibitors was critical to the use of these assays. Langer et al., 1976, Nature 263:797-800.
In the CAM bioassay, fertilized chick embryos are cultured in Petri dishes. On day 6 of development, a disc of a release polymer, such as methyl cellulose, impregnated with the test sample or an appropriate control substance is placed onto the vascular membrane at its advancing edge. On day 8 of development, the area around the implant is observed and evaluated. Avascular zones surrounding the test implant indicate the presence of an inhibitor of embryonic neovascularization. Moses et al., 1990, Science, 248:1408-1410 and Taylor et al., 1982, Nature, 297:307-312. The reported doses for previously described angiogenesis inhibitors tested alone in the CAM assay are 50 .mu.g of protamine (Taylor et al. (1982)), 200 .mu.g of bovine vitreous extract (Lutty et al., 1983, Invest. Opthalmol. Vis. Sci. 24:53-56), and 10 .mu.g of platelet factor IV (Taylor et al. (1982)). The lowest reported doses of angiogenesis inhibitors effective as combinations include heparin (50 .mu.g) and hydrocortisone (60 .mu.g), and B-cyclodextrin tetradecasulfate (14 .mu.g) and hydrocortisone (60 .mu.g), reported by Folkman et al., 1989, Science 243:1490.
According to the rabbit corneal pocket assay, polymer pellets of ethylene vinyl acetate copolymer ("EVAC") are impregnated with test substance and surgically implanted in a pocket in the rabbit cornea approximately 1 mm from the limbus. Langer et al., 1976, Science 193:707-72. To test for an angiogenesis inhibitor, either a piece of carcinoma or some other angiogenic stimulant is implanted distal to the polymer 2 mm from the limbus. In the opposite eye of each rabbit, control polymer pellets that are empty are implanted next to an angiogenic stimulant in the same way. In these control corneas, capillary blood vessels start growing towards the tumor implant in 5-6 days, eventually sweeping over the blank polymer. In test corneas, the directional growth of new capillaries from the limbal blood vessel towards the tumor occurs at a reduced rate and is often inhibited such that an avascular region around the polymer is observed. This assay is quantitated by measurement of the maximum vessel lengths with a stereospecific microscope.
Troponin, a complex of three polypeptides is an accessory protein that is closely associated with actin filaments in vertebrate muscle. The troponin complex, acts in conjunction with the muscle form of tropomyosin to mediate the Ca.sup.2+ dependency of myosin ATPase activity and thereby regulate muscle contraction. The troponin polypeptides T, I, and C, are named for their tropomyosin binding, inhibitory, and calcium binding activities, respectively. Troponin T binds to tropomyosin and is believed to be responsible for positioning the troponin complex on the muscle thin filament. Troponin I binds to actin, and the complex formed by troponins I and T, and tropomyosin, inhibits the interaction of actin and myosin. Troponin C is capable of binding up to four calcium molecules. Studies suggest that when the level of calcium in the muscle is raised, troponin C causes troponin I to loose its hold on the actin molecule, causing the tropomyosin molecule shift, thereby exposing the myosin binding sites on actin and stimulating myosin ATPase activity. Prior to the discovery of the present invention, troponin subunits were not known to inhibit the process of endothelial cell proliferation.
The citation of a reference herein shall not be construed as an admission that such reference is prior art to the present invention.